Ubiquitous coverage such that wireless communication can be held anywhere in the world, which is an ability of communication terminal equipment, such as mobile phone terminals, has not been a reality today, but is under development.
Such mobile systems include GSM, GPRS, EDGE, WCDMA, DCS and PCS cellular systems. As to the characteristics of these systems, the demands for multiband and multimode systems, which are arranged by widespread combinations of signals with a fixed envelope and signals with a changing envelope, and of time-division multiplex and code-division multiplex, have been growing. Now, it is noted that GSM stands for “Global System for Mobile Communication”, GPRS for “General Packet Radio Service”, EDGE for “Enhanced Data for GSM Evolution” or “Enhanced Data for GPRS”, WCDMA for “Wideband Code Division Multiple Access”, DCS for “Digital Cellular System”, and PCS for “Personal Communication System”.
A typical two-step transmitter is described in a cited reference presented by Abdellatif Bellaouar, “RF Transmitter Architectures for Integrated Wireless Transceivers”, The Eleventh International Conference on Microelectronics, 1999, 22-24 Nov. 1999, pp. 25-30. The typical two-step transmitter has: a quadrature modulator including mixers, a π/2-phase divider, and an adder: a first band-pass filter; an RF mixer; a buffer amplifier; and a second band-pass filter, in which an output signal from the second band-pass filter is supplied to an RF power amplifier. In such a two-step transmitter, baseband signals I and Q are fed to one input terminal of the two mixers of the quadrature modulator; an intermediate-frequency local signal is supplied to an input terminal of the π/2-phase divider; and two output signals of the π/2-phase divider differing in phase by π/2 (90 degrees) are supplied to the other input terminals of the two mixers. Two output signals from the two mixers are supplied to two input terminals of the adder. Thus, the baseband signals are converted up by the intermediate-frequency local signal to an intermediate frequency of, e.g., 70 MHz. Between an output terminal of the adder of the quadrature modulator and one input terminal of the RF mixer is connected the first band-pass filter for removing harmonics with the intermediate frequency. To the other input terminal of the RF mixer, an RF (Radio Frequency) local signal is supplied. An RF output signal from the RF mixer is amplified by the buffer amplifier, and then fed to the second band-pass filter for removing undesired sidebands. The solution of using a filter to attenuate sidebands of high levels is a very simple and low-power measure, however it is hard to realize such a filter and a physically large off-chip size is required. Further, in the cited reference presented by Abdellatif Bellaouar, a direct up-conversion architecture is also introduced, which can be realized by a smaller number of devices. According to the architecture, baseband signals are directly converted into RF transmit signals with RF local signals supplied to the π/2-phase divider of the quadrature modulator, and the resultant signals arise at an output of the adder of the quadrature modulator.
In another cited reference presented by Gabriel Brenna et al, “A 2-GHz Carrier Leakage Calibrated Direct-Conversion WCDMA Transmitter in 0.13-·m CMOS”, IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 39, No. 8, AUGUST 2004, pp. 1253-1262, a DUC architecture is introduced as a hopeful candidate for a highly integrated transmitter in that costly external parts are eliminated. Herein, DUC stands for direct up-conversion. According to the DUC transmitter architecture, a transmit baseband signal I/Q is supplied to an I/Q modulator through a baseband filter. The I/Q modulator converts the transmit baseband signals to a radio frequency (RF). At the radio frequency, the signals I and Q are synthesized and amplified. After going through external filtering and additional amplification, the resultant signal is supplied to a duplexer, and then launched through an antenna. To reduce the oscillator frequency pulling into outputs of a power amplifier (PA), the local oscillator (LO) is set to 4 GHz, which is double the carrier frequency. A digital divider is used to generate a precise 2-GHz quadrature local signal.
Further, according to the cited reference presented by Gabriel Brenna et al., carrier leakage is pointed out as a serious drawback which DUC architecture has. The carrier leakage produces interference signals of about 1.9 GHz, which fall in the frequency band of WCDMA signals ranging from 1.895 to 1.905 GHz. The carrier leakage can cause EVM (Error Vector Magnitude) and ACPR (Adjacent Channel Power Ratio) to exceed the values stated in the specifications. According to the cited reference presented by Gabriel Brenna et al., offset calibration is adopted in order to suppress the carrier leakage, in which two operational amplifiers of the baseband filter use a 6-bit current source. Also, according to the cited reference presented by Gabriel Brenna et al., carrier leakage calibration is embraced to suppress carrier leakage, in which a 5-bit binary weighted current source is used for the I/Q modulator. The power of carrier leakage is detected by an on-chip power detector with no transmit signal. Analog output voltages from the detector are converted into digital signals by use of an automatic digital calibration algorithm. The algorithm controls a calibration circuit for the modulator and baseband filter so as to minimize the measured carrier leakage.